In large compression ignition internal combustion engines, such as those used in ships or power plants, smoke formation at low loads is a common problem. The fuel injectors of the engines must be capable of delivering the fuel amount needed for the operation at maximum load. Since large internal combustion engines need to run reliably and efficiently at maximum or near maximum power, the injectors are even optimized for high loads. This means that at low loads the injectors do not work optimally and smoke is formed.
The problem has been solved in prior art solutions by using injectors with two needles. For instance, in European patent application EP 0972932 A1 is disclosed a fuel injector comprising several nozzle drillings. For a part load operation, only the outer valve needle is moved for revealing nozzle drillings in an upper region. For higher load, also the inner valve needle is moved for revealing nozzle drillings in a lower region.
In patent application US 2007246561 A1 is described a fuel injector comprising two nozzles that are configured for providing different fuel spray patterns. One of the nozzles is optimized for conventional injection, whereas the other nozzle is optimized for use with homogeneous charge compression ignition (HCCI).
Another problem of internal combustion engines is NOx formation, which occurs especially at high combustion temperatures. NOx emissions can be removed effectively from the exhaust gases with selective catalytic reduction, but it is desirable to be able to reduce the NOx formation already in the first place. NOx formation can be best reduced by lowering cylinder temperatures. One way of doing this is to apply the so called Miller cycle, which utilizes modified intake valve closing process. There are two versions of the Miller cycle: In the first version, the intake valve is closed only after bottom dead center (BDC), which means that part of the compression stroke is not used for compression. This is often used in spark ignition engines to achieve higher thermal efficiency and a lower tendency to knocking. In the other version of the Miller cycle, early closing of the intake valve is utilized. When the intake valve is closed before the piston arrives at bottom dead center, pressure and temperature in the cylinder will be lower at the end of the compression stroke. This version of the Miller cycle is often used in compression ignition engines. The smaller amount of air in the cylinder due to the early closing of the intake valve is usually compensated by an adequate supercharge ratio.
However, at part and low loads the use of the Miller cycle increases NOx emissions. Also, the optimal Miller cycle with respect to the NOx emissions gives higher smoke emissions. Also the specific fuel oil consumption (SFOC) is higher at low loads if the Miller cycle is used.